1. Field of the Invention
The present invention relates generally to sealing methods and apparatus and more particularly to a method and apparatus for sealing the open ends of pipettes constructed of plastic materials.
2. Discussion of the Prior Art
Pipettes are well know devices used in serological, biological and chemical laboratories for delivering small volumes of liquid, such as blood serum, into another container or onto a prepared surface.
Until recently, pipettes were constructed of a glass tube having a small diameter connected with a flexible rubber bulb to produce a vacuum by squeezing to draw the liquid into the glass tube. Disposable pipettes have been molded from a plastic material, such a polyethylene, and consist of a thin plastic stem integral with a plastic bulb at one end. The pipette is a single piece of plastic having an opening at the end of the stem and the stem is inert to various liquids dispensed by the pipette. The bulb is constructed of thin enough material so that it can be squeezed between the thumb and forefinger and will promptly return to original shape. Various designs of pipettes and the methods of making the same are disclosed in U.S. Pat. No. 4,212,204 issued to the present inventor.
After a liquid sample has been drawn into the pipette, it is often desirable to seal the open end of the stem to protect the sample from contamination and to prevent leakage or inadvertent dispensing of the liquid. Various types of mechanical crimping devices have been suggested for this purpose. However, many of these devices are cumbersome, difficult to use and frequently fail to completely seal the pipette from atmosphere.
Attempts have also been made to develop devices for sealing the pipette stem using heat. Typically, such devices embody cooperating metal jaws, or dies, which are heated and then moved into pressural engagement with the pipette stem. The patent to Allisbaugh, U.S. Pat. No. 3,166,940 discloses in FIG. 13 a heated metal die construction adapted for use in shaping the end of a cellophane tube.
Devices which use heated metal sealing jaws or dies are generally unsuitable for use in sealing pipettes constructed from plastics such as polyethylene because the plastic will stick to the heated metal resulting in imperfect seals, and often times unacceptable deformation and damage to the pipette stem. To attempt to avoid this undesirable sticking of the plastic to the heated metal, lubricants such as silicone grease have been applied to the pipette or to the die. This approach is undesirable because of the inconvenience encountered and the possibility of the lubricant contaminating the sample contained within the pipette.
The present invention effectively overcomes the drawbacks of prior art sealing devices by providing a novel apparatus in which a heated fluoroplastic is used to accomplish the tube sealing function. More particularly the device of the present invention embodies a uniquely configured fluoroplastic sealing head having a shaped tube receiving cavity and means for controllably heating the sealing head to a predetermined temperature in excess of the softening temperature of the plastic material from which the pipette is formed.